WO1987001716A1

WO1987001716A1 - Latent heat storage medium and use thereof

Info

Publication number: WO1987001716A1
Application number: PCT/DE1986/000369
Authority: WO
Inventors: Gerd Hörmansdörfer
Original assignee: Hoermansdoerfer Gerd
Priority date: 1985-09-18
Filing date: 1986-09-17
Publication date: 1987-03-26
Also published as: JP2736379B2; DE3674191D1; US4795580A; AU6339086A; EP0236382A1; JPS63500946A; EP0236382B1

Abstract

Latent heat storage medium based on polyvalent aliphatic alcohols of the general formula (I) where n = 4, 6 or 8 (preferably 4 or6) and with such a configuration isomerism that there is created a configuration which is symmetrical about the middle of the chain. Preferably two or more of these alcohols are used mixed together, whereby the melting temperature and/or the supercooling temperature of the medium can be regulated to a specific value by choosing the ratio of the mixture, which can be eutectic, hypoeutectic or hypereutectic. The proposed latent heat storage media are ideal for use in heat protection systems for measurement probes used in geotechnic bore holes. They cover the required temperature range, are marked by unusually high volume/specific melting enthalpy at moderate densities, are cycle-resistant and exhibit low heat conductivity. The boiling points are often significantly above 300oC, and their temperature resistance extends substantially into the boiling point range. In addition, construction materials are not attacked by them, and also their physiological safety is guaranteed.

Description

Latent heat storage medium and its use

The invention relates to a latent heat storage agent based on polyhydric aliphatic alcohols of the general formula

d. H. those with an unbranched hydrocarbon chain and one hydroxyl group per carbon atom. The invention further relates to a preferred use of this agent.

Latent heat storage means are used in numerous heat transfer processes as means for heat storage. They have a melting point in the range of the working temperature of the heat transfer process, so that they change from the solid to the liquid state when heated and return to the solid state when they cool down, thus also absorbing latent heat (heat of fusion) and deliver again. Compared to so-called capacity heat storage means (ie those in which the state of matter does not change in the range of the working temperature, so that only sensible heat can be absorbed and released again), latent heat storage means thus have the Advantage that they allow the storage of a larger amount of energy in a narrow temperature range with a relatively small volume requirement.

In general, latent heat storage means should have a melting enthalpy that is as high as possible, the volume-specific (ie volume-related) melting enthalpy being particularly important in order to create a maximum amount of storage capacity per unit volume of the storage system. They must also be cycle-stable, i.e. H. The solid-liquid-solid phase transition must remain reversibly reproducible over long periods of time and must not be impaired by chemical reactions, segregation or the like. Other important criteria can also be the solidification behavior (e.g. the formation of a metastable melt, the extent of a change in volume during the phase transition or the form of crystallization), the compatibility with the construction materials, the physiological harmlessness and the Availability at an acceptable price. It has so far proven difficult to find latent heat storage means in which all of these criteria are met in an optimal manner for the respective application.

Many of the latent heat storage media known to date have been developed for use in the area of space heating and accordingly have melting points in the hot water area. In addition to inorganic substances; (which are often corrosive and low cycle ^* fes ^* t) as salt hydrates and Salzmischun¬ gen are already organic substances including individual Alko¬ this proposed hole. For example, DE-A 28 56 374 describes the use of simple alcohols such as stearyl alcohol, hexadecanol or octanediol as latent heat storage means for solar heating systems. However, the volume-specific enthalpy of fusion of these simple alcohols is comparatively low because of their low density. Furthermore, DE-A 27 4 664 describes the use of xylitol (that is an alcohol according to the general formula A mentioned at the beginning with n = 5) as latent heat storage medium for heat packs or for heating systems that work with solar energy or with night electricity. Compared to the simple alcohols, xylitol has a higher volume-specific melting enthalpy, but forms a pronounced metastable melt on cooling, which is still stable even at room temperature and can only be crystallized by additional measures such as seeding, rubbing or exposure to ultrasound. This may be expedient for the applications provided for in DE-A 27 47 664, but it is not in most other cases.

A special other application of latent heat storage means is the use in heat protection systems for measuring probes in deep geotechnical boreholes. The point here is to, during the metrological inspection of a borehole at mud temperatures up to 200-300 ° C, an 'electronic measuring insert, which is inserted into a heat shield designed as a Dewar vessel, over a long period of time at a low temperature (e.g. E.g. between 70 and 125 ° C). Usually this will be in front of and behind the mounted tentwärmesenken Meßeinschub bullet-shaped laser ^', which are filled with the latent heat storage medium and have to 2 liters usually a total volume of only about 0.5.

This application makes demands on the latent heat storage means which cannot be met by the latent heat storage means known in the area of space heating. Up to now, therefore, molten metals, e.g. B. Wood's metal, used as a latent heat storage agent. However, molten metals have the disadvantage of a high density, which is between 9 and 10 kg / dm and leads to a high total weight of the latent heat sink. As a result, the relatively thin inner wall of the heat shield is particularly endangered by vibrations. This cannot be counteracted by a reinforced inner wall of the heat shield, since its insulation value will then deteriorate. In addition, a recent study of such heat protection systems has - 4 -

(LA-9671-HDR, GA Bennett, GR S erman, Analysis and Thermal Design Improvements of Downhole Tools for Use in Hot Dry Wells, Los Alamos National Laboratory) show that the thermal conductivity inherent in the melting metals is also a serious disadvantage, because it transports an increased heat flow into the interior of the heat protection system via the latent heat sinks. Furthermore, in this study, an even higher melting enthalpy of the latent heat storage medium to be used than the melting metals is mentioned as a prerequisite for a satisfactorily operating system. The weight-related melting enthalpy of the melting metals is in fact relatively low, so that despite the high density, the values for the volume-specific melting enthalpy remain unsatisfactory. This means that the same disadvantageous conditions are present as in the reverse case of a higher weight-related melting enthalpy with a very low density of a latent heat storage medium.

There is therefore a need for an improved latent heat storage medium, in particular for borehole heat protection systems, which has the highest possible volume-specific melting enthalpy at melting points in the range between approximately 50 and 150 ° and moderate densities between approximately 1 and 2 kg / dm ³ , a low thermal conductivity possesses resistance even at high temperatures (as far as possible up to the range of 300 ° C.) and, moreover, the requirements for cycle stability, solidification behavior, compatibility with the construction materials, physiological harmlessness and availability as possible optimally fulfilled.

The invention provides such an improved latent heat storage agent, specifically according to the invention based on polyhydric aliphatic alcohols of the general formula

H 1

H - C - H OH ^J n (A) - 5 -

with n = 4, 6 or 8 and with such a configuration isomerism that there is a symmetrical configuration with respect to the chain center.

The invention is based on the surprising finding that alcohols of the general formula (A) are outstandingly suitable as latent heat storage agents, in particular for borehole heat protection systems, if their chain length is even and their configuration is symmetrical with respect to the chain center. It has been found that the alcohols proposed according to the invention as latent heat storage agents do not only have one in comparison to the alcohols with an odd chain length (e.g. xylitol with n = 5) and / or asymmetrical configuration, which also fall under the general formula (A) have significantly higher melting enthalpy, but also have a significantly lower supercooling of the melt and above all have the property of reproducibly and voluntarily transitioning to the crystalline state during cooling at a specific fixed supercooling temperature. These phenomena were unpredictable and cannot yet be scientifically explained.

Of the alcohols proposed according to the invention as latent heat storage agents, those with n = 4 or 6, ie the tetrites and hexites with a symmetrical configuration, are preferred. These are erythritol and threit with n = 4 and dulcitol, allitol, iditol and mannitol with n = 6. The configuration of these preferred alcohols, as illustrated in the attached Table 1, can either be mirror-symmetrical (erythritol, dulcitol and allitol) or be axially symmetrical (threit, idit and mannit), each related to the chain center. Table 2 also shows the data for the most important of these preferred alcohols.

The alcohols proposed according to the invention as latent heat storage agents are preferably used not as pure substances but as a mixture of two or more substances. set, where expediently at least one mixture component should have mirror symmetry. It has been found that the melts of the alcohols proposed according to the invention are miscible with one another in any ratio and form cooling eutectics which, depending on the number of mixture components, are binary, ternary or higher and for which some examples are also shown in Table 2 are specified. By using such mixtures, which can be eutectic, hypereutectic or hypoeutectic, lower melting temperatures can be set which are generally desirable in the interest of minimizing the thermal load on the measuring probe. In addition, the mixtures form a finer and more uniform crystal structure than the pure substances, with the addition of a small amount of z. B. 0.5 mol% of a mixture component advantageously influences the crystal structure.

Another important advantage of the mixtures results from their hypothermia behavior. In general, a certain degree of hypothermia, ie the formation of a metastable melt, is desirable for latent heat storage agents which are to be used as temporary heat protection, because the heat of crystallization is only released again after a delay. This hypothermia is relatively low for the pure substances; (up to about 20 K) has a maximum value for eutectic mixtures and runs almost linearly with the mixing ratio in the case of hypoeutectic or hypereutectic mixtures, as shown in FIG. 1 using the example of the dulcite / mannitol system. There curve 1 shows the course of the melting temperature and curve 2 the course of the supercooling temperature (ie the temperature at which the metastable melt reproducibly and voluntarily changes into the crystalline state) in each case as a function of the mixing ratio. By selecting the mixing ratio, the desired subcooling can be set precisely, expediently such that the subcooling temperature according to curve 2 is not below about 50 ° C. This ensures that the melt with normal - 7 -

exercise temperature has definitely changed into the crystalline state and that it can be checked by hand simply by touching the latent heat sinks whether this is the case.

Overall, the alcohols proposed according to the invention as latent heat storage means ideally meet the requirements for use in heat protection systems of borehole probes. They cover the required temperature range, are characterized ^* by unusually high volume-specific melting enthalpies with moderate density, are cycle-resistant and have a low thermal conductivity. Their boiling points are in some cases considerably above 300 ° C., the temperature resistance essentially extending into the boiling point range, and their supercooling can be set in a targeted manner. In addition, construction materials are not attacked by them and the physiological harmlessness is also guaranteed. They now allow the production of thermal protection systems with a significantly improved service life and with significant additional advantages that were previously not possible.

- 8th -

Table 1

Configurations

Mannit Allit Idit Dulcit

CH, 0H CH, OH CH, OH CιH-OH

HO c - H H - C | - OH HO - C - H H - C - OH

HO c - H H - C - OH H - C - OH HO - C - - H

H c - OH H - C - OH HO - C | - H HO - C | - - H

H C | - OH H - C - OH H - C - OH H - C | - - OH

CH ₂ 0H CH ₂ OH CH ₂ OH CH ₂ OH

Erythritol Threit

CH ₂ 0H CH ₂ 0H

H - C - OH OH - C i I - H H - C - OH H - C - OH

CH ₂ 0H CH ₂ OH

_ 9 _

Table 2

Data

Substance Melting point Density of enthalpy of fusion

(° C) (kg / dm ³ ) (kJ / kg) (kJ / dm ³ )

Threit (T) 71 1.46 223 326

Erythritol (E) 119 1.46 334 488

Mannitol (M) 167 1.50 306 459

Dulcit (D) 189 1, 50 358 537

Eutectics *)

M / D 50:50 153 1.50 282 423

E / M 80:20 113 1.50

E / D 80:20 115 1.50

E / M / D 80: 10: 10 109 1.50 274 411

*) The numbers indicate the molar ratio of the eutectic mixture.

Claims

- 10-patent claims

1 . Latent heat storage agent based on polyvalent aliphatic alcohols of the general formula

2. Latent heat storage medium according to claim 1. the configuration of the alcohols being mirror-symmetrical with respect to the chain center.

3. Latent heat storage medium according to claim 1, wherein the configuration of the alcohols is axis-symmetrical with respect to the chain center.

4. Latent heat storage agent according to claim 2, wherein the alcohol is erythritol (n = 4).

5. Latent heat storage agent according to claim 2, wherein the alcohol is dulcite or allite (n = 6).

6. Latent heat storage agent according to claim 3, wherein the alcohol is Threit (n = 4).

7. latent heat storage medium according to claim 3, wherein the alcohol is idit or mannitol (n = 6). - 1 1 -

8. Latent heat storage medium according to one of the preceding claims, wherein two or more of the alcohols are present in a mixture with one another.

9. Latent heat storage medium according to claim 8, wherein at least one of the alcohols present in the mixture has a mirror-symmetrical configuration.

10. Use of the latent heat storage means according to one or more of claims 1 to 9 in heat protection systems for measuring probes in deep geotechnical drilling.